With the discovery of conducting polymers about ten years ago, the possibility of combining the important electronic properties of semiconductors and metals with the attractive mechanical properties and processing advantages of copolymers was proposed. Without exception, however, the initial conducting polymer systems were insoluble, intractable, and non-melting (and thus not processable) with relatively poor mechanical properties. Specific examples of intractable conducting polymers with attractive electronic properties are polyacetylene ((CH).sub.x), and polyparaphenylene (C.sub.6 H.sub.4).sub.x. These two systems have the highest density of pi electrons, and both can be doped either p-type (oxidation) or n-type (reduction). Other well-known examples are "polyaniline", or poly(paraphenyleneimineamine) (PPIA), and polypyrrole, which are air stable.
The class of conducting polymers has been enlarged, and a good understanding of the fundamental molecular features which are necessary to achieve and control the electronic properties of these polymers has begun to develop. Soluble conducting polymers have been developed (i.e., soluble either in water or in common organic solvents), and initial attempts at processing from solution have proven successful. Major improvements have been made in material quality and in environmental stability as well as in the achievement of highly oriented (chain-aligned) materials. Based on this progress, there is every reason to believe that these materials will continue to evolve to the point where they can be used in a wide variety of technological applications. A number of potentially important application areas have already been identified, including use as anisotropic electrical conductors, use in novel electrochemical applications, and use in the exploitation of nonlinear optical phenomena.
Progress has been made toward rendering specific systems soluble and thereby processable. For example, the poly(3-alkylthiophene) derivatives (P3ATs) of polythiophene are generally soluble and have been processed into films and fibers. See, e.g., Hotta, S., et al., Macromolecules 20:212 (1987); Nowak, M., et al., Macromolecules (in press); Hotta, S., et al. Synth. Met. (in press); Elsenbaumer, R. L., et al. Synth. Met. 15:169 (1986); Polym. Mat. Sci. Eng. 53:79 (1985); and Sato, M., et al., J. Chem. Soc. Chem. Commun. 83 (1986). However, the enhanced solubility was achieved by adding relatively long alkyl chains onto the polymer backbone, on the 3-position of the thiophene ring. The recently discovered water-soluble self-doped conducting polymers (see Patil, A. O., et al., J. Amer. Chem Soc. 109:1858 (1987); and Patil, A. O., et al., Synth. Met., in press) also achieve solubility through functionalizing at the 3-position of the thiophene ring with the relatively bulky (CH.sub.2).sub.n SO.sub.3 group. Other related examples exist.
Although important, these soluble conducting polymers also have a number of inherent disadvantages. Films, fibers, and the like formed from these polymers have a lower density of pi electrons. They also have reduced interchain electron transfer integrals, and the monomers have a higher molecular weight due to the addition of the flexible side group. Since the density of pi electrons is a critical parameter in determining the electronic properties of a material, ranging from the material's electrical conductivity to its nonlinear optics, the pi electron density should be maximized. Also, because macroscopic electrical properties are limited by the ability of electrons to move from chain to chain, interchain transfer should actually be optimized. As the energy density of a polymer battery electrode decreases with the addition of nonconjugated side-groups, these soluble systems will inherently have lower energy density. As a result, the electronic properties of the soluble conducting polymers are quite limited relative to the aforementioned intractable systems. For example, the higher electrical conductivity reported for doped films of the P3ATs is about 100 S/cm, whereas unoriented polyacetylene can be prepared with a conductivity of at least 2000 S/cm; for oriented materials, values in excess of 10.sup.5 S/cm have been reported (Naarmann, H., Synth. Met. 17:2233 (1987); Naarmann, H., Symposium on "Conducting Polymers: Their Emergence and Future", Amer. Chem. Soc. Meeting, Denver, Colo., Apr. 8-9, 1987, in which conductivity of 1.5.times.10.sup.5 S/cm was reported for iodine-doped polyacetylene). Thus, although processing from solution offers some advantages, it has limited applicability within the class of conducting polymers, as degradation of many electronic properties would result.
As an alternative to the soluble conducting polymer systems, shaped articles such as fibers, tapes and the like fabricated from intractable conducting polymers are generally unknown, and yet would be highly desirable in a wide range of technological applications. If, in addition, such shaped articles could be made with the polymer chains highly oriented along the draw direction, the resulting conducting polymers would be expected to be highly anisotropic with the desired electrical, optical, and nonlinear optical properties principally along the orientation direction.
To this end, alternative methods of synthesizing polyacetylene have been explored. The synthesis developed at Durham University (so-called Durham polyacetylene) offers particular advantages in that a soluble and processable prepolymer is converted to polyacetylene as a final step (Feast, W. J. in ref. 1, Chapter 1, vol. 1; Bott, D. C., et al., Mol. Cryst. Liq. Cryst. 117:95 (1985); Kahlert, H., et al., Mol. Cryst. Liq. Cryst. 117:1 (1985)). The Durham polyacetylene can be prepared as amorphous material, or it can be stretch-oriented simultaneously with conversion and isomerization into highly anisotropic free-standing fibers or films. A gel phase of the processable precursor polymer has been used to produce fibers which were subsequently converted to polyacetylene. The resulting materials, however, had limited utility (e.g., maximum conductivity after doping of only 30 S/cm). Moreover, although the resulting Durham polyacetylene can be made so that it is a highly oriented material, it has specific disadvantages which limit its potential utility. The fibrillar morphology is not present in Durham polyacetylene; this material has a high density with no microstructure visible by electron microscopy. Consequently, the doping kinetics of Durham polyacetylene are extremely slow, limiting the areas of potential application. Furthermore, the electrical conductivity of the resulting material after doping is limited; maximum values in the literature are below 1500 S/cm.
Thus, the ability to fabricate electroactive polymers into shaped articles such as fibers, films and the like remains seriously limited.